(a) Field of the Invention
The present invention concerns the preparation of ultra-pure nitrogen, having an amount of residual impurities (essentially CO, H.sub.2, CO.sub.2, H.sub.2 and O.sub.2) lower than a few tens of ppb (parts by billion), generally less than 10 ppb, and, even still less, of the order of 1 ppb.
(b) Description of Prior Art
Pure nitrogen is generally obtained by cryogenic distillation of air, which enables to separate nearly all of the oxygen, while carbon dioxide and water vapor have previously been removed by being blocked on an adsorbent before distillation.
However, carbon monoxide (CO) and hydrogen (H.sub.2) which are present in the air are found again in nitrogen after distillation, unless there is provided, in the case of hydrogen, the addition of a complementary distillation column, which has been found to be particularly costly.
This is the reason why, presently, air is pretreated after compression and before being sent towards the drying and decarbonating device, by catalytic means utilising precious metals such as platinum and palladium supported on alumina and at elevated temperature, above 100.degree. C. or in the case of carbon monoxide, copper oxide CuO at a temperature of the order of 150.degree. C., after which, the air thus purified can be sent towards the distillation column, but this operation at elevated temperature which is carried out on the entire flow of air is also costly and it is difficult to achieve the aimed limit of 10 ppb, for carbon monoxide as well as for hydrogen, in which the residual amounts remain of the order of 100 ppb.
In the document U.S. Pat. No. 4,869,883, it has been proposed to use purifiers which treat nitrogen at elevated temperature to remove CO, CO.sub.2, H.sub.2 O, O.sub.2, H.sub.2, and which operate by removing oxygen by reaction with CO and/or H.sub.2 with Cu with production of CO.sub.2 and/or H.sub.2 O, by removal of CO and/or H.sub.2 on CuO, followed by removal of CO.sub.2 and H.sub.2 O on molecular sieves, possibly preceded by a bed of alumina. Such purifiers are relatively complicated to operate and, for example, require a large number of high pressure reservoirs at elevated temperature. A process of the same type operating at room temperature on a nickel base catalyst (NiO and Ni with about 50% by weight of Ni with at least 5% by weight of Ni in the form of Ni metal) is described in the document E.P.-A-240,270, which enables to reduce the amount of impurities to below 100 ppb. It has the disadvantage of requiring a regeneration utilising a gas containing hydrogen. Generally, attempts have even been made to block CO and H.sub.2 by adsorption, however the adsorbents or known sieves do not operate for H.sub.2 and are not very efficient for CO. For example, the 13X molecular sieve blocks CO.sub.2 but very little CO. However, it is known that the efficiency of these absorbents may be improved by exchange of copper ions (Cu) or impregnation of palladium (Pd), but no proposal has yet been made to adapt this technique to the preparation of ultra-pure nitrogen.